A recurring problem in ink jet printers is the accidental and undesired contact of the ink jet with the charging electrodes used to charge the jet before or while a drop is being formed in the jet. Such contact has been extremely detrimental in the past, because (a) with some printer designs, this shorts out the electrode and shuts down the printer, and (b) it causes an electrolytic degradation of the metal of the electrode, e.g., dissolution of the metal of the electrode.
To cure this problem, it has not been enough to simply control the purity of the jet. Although ink purity and solubility are factors that affect jets, sooner or later even the cleanest and best maintained printer head experiences some wandering of the jets, particularly in light of the fact that the jet, under some conditions, is as close as 68 .mu.m to the charging electrodes.
Thus, attempts have been made to apply a thin dielectric coating to the electrodes, to protect them against contact with the ink. A related use of such dielectrics is that described in patents such as U.S. Pat. No. 3,789,278, wherein semiconductive materials such as undoped amorphous silicon, SiO.sub.2, Si.sub.3 N.sub.4, BN and others are described for use in creating a corona discharge. However, such undoped materials have been found to be unsatisfactory in ink jet printers, in that, while protecting the electrodes, they have such a high resistance that the charges formed on the dielectric coating from the ionized air adjacent thereto, build up and interfere with the desired charging of the passing drops. More specifically, the unwanted surface charges on the dielectric cannot dissipate into the charging electrode fast enough to permit the charging of sequential drops which pass by as fast as 10 microseconds apart.
Thus, when read as a whole, it is not true that the aforesaid '278 patent teaches the resistivity need only be .gtoreq.10.sup.6 ohm-cm, notwithstanding the statement in col. 3, line 27. Instead, the materials actually described have a much higher room temperature undoped resistivity. For example, Al.sub.2 O.sub.3 =10.sup.13 -10.sup.15, MgO=1.3.times.10.sup.15, Si.sub.3 N.sub.4 =10.sup.13, SiO.sub.2 =10.sup.21, amorphous silicon=10.sup.12, BN=10.sup.16, and Ta.sub.2 O.sub.5 =10.sup.14 ohm-cm. Although ZnO is 10.sup.10 in some cases, it is soluble in some ink and thus unsatisfactory for the instant invention.
The problem then prior to this invention has been to find a way of protecting the charge electrodes from electrolysis damage due to contact with the ink, and at the same time allow ionization charges from the air, to leak back to the charging electrodes.